chore: cleanup some parts of qmagicenum

[Nerixyz]

• Removes enable_ifs over concepts (return types are a bit more readable this way)
• Ports magic_enum's is_nothrow_invocable to predicates over QChar
• Adds some comments to the storage
• Moves static string construction to detail
• Removes usage of magic_enum::detail::enum_subtype - this is resolved by magic_enum (except for enumFlagsName, where we verify the type)
• Removes inline for constexpr functions
• Adds two more members to the flags test that live outside the regular enum range (-128…128)